A grate assembly for a fluidized bed boiler of the above-described type is described in Finnish patent application FI-935455. The grate assembly described in this patent application has proved to be very functional in practice, in particular with regard to cooling the grate assembly. It has been noticed in practice that for cooling of the sparge pipe it is advantageous to position the cool medium circulation at least partially at the upper edges of the sparge pipes in such a manner that the cool medium channel of the cool medium circulation, in particular the cooling duct, is arranged in two adjacent sparge pipes to be situated at the upper edges of the aperture system. An implementation of this type of cool medium circulation is shown in FIG. 3 of Finnish patent application FI-935455. Thus, the edge area which is critical in view of the endurance of the sparge pipe, is cooled, wherein no high heat tensions are effected thereto. This applies in particular to the corner area where the substantially horizontal upper surface of the cross section of the sparge pipe is changed to a substantially vertical side wall of the sparge pipe, i.e., to the substantially vertical side edge of the aperture system. On the other hand, this structure, advantageous with regard to cooling of the sparge pipe, involves problems in particular prior art solutions, the means for supplying fluidizing air have to be placed, in particular as to the air supply location, i.e. the junction between the upper surface of the sparge pipe and the means for supplying fluidizing air, further away from the aperture system, to the upper surface of the sparge pipe, perpendicularly to the longitudinal direction of the aperture system. This is effected specifically by the fact that the cool medium circulation is placed, at least partially, at the upper corners of the sparge pipes. However, it should be noted that in view of the operation of the fluidizing process, the fluidizing air has to be distributed evenly to the fluidized bed situated above the grate assembly. In other words, the entire fluidized bed has to be kept in a fluidized phase. So-called coarse material is accumulated specifically at such places in the fluidized bed where the air blow is insufficient. In case a process condition of this kind is effected at the aperture system in the grate assembly that is composed, at least partially, of sparge pipes, the danger exists that the aperture system chokes owing to the fact that there is no air blow or an insufficient air blow at the aperture system. Consequently, larger sintered pieces formed of coarse material are produced at the aperture system, and it is most probable that these pieces will eventually block the aperture system, at least partially. It is obvious that the air supply could be effectuated to be smooth and sufficient for maintaining a fluidized phase in the fluidized bed by increasing the air blow through nozzles or by enlarging the nozzle apertures, but in that case it is most probable that excessive air blows have to be introduced, which may cause disturbances to the process itself. In any case, excessive air blows increase the energy consumption of the process.
The above described drawbacks have given rise to the present invention which further improves and raises the level of technology related to cooled grate assemblies composed of sparge pipes and used in fluidized bed boilers. A particular purpose of the present invention is to ensure a smooth supply of fluidizing air for maintaining the fluidized bed, in particular in such the area of the aperture system in a manner that the energy costs are reasonable and the aperture system is not choked.